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Preheat the oven to 175°C. Line a 9-inch square pan with a piece of aluminum foil (about 17 inches long), leaving a 2-inch overhang on each side. Lightly coat the bottom and sides of the lined pan with cooking spray.

Melt half the butter (4 tablespoons) in a medium bowl in the microwave, about 45 seconds. Set aside 1 cup of the cake mix, and add the remaining mix to the melted butter. Add the egg and 1 tablespoon water, and mix with a spatula until well blended. Let stand a few minutes until the mixture is no longer tacky to the touch. Pat the dough in an even layer in the pan. Bake until the edges are lightly set to the touch and the crust is lightly browned all over, about 20 minutes. Let cool for 5 minutes.

Meanwhile, melt the remaining butter in a medium bowl in the microwave, about 45 seconds. Add the reserved cake mix, oats and cinnamon, and mix with a fork until crumbly.

Spread the apple butter over the cooled crust, spreading it to the edges. Scatter the crumb mixture over the apple butter. Return the pan to the oven and continue to bake until the crumbs are golden brown and firm, about 30 minutes more. Cool completely in the pan.

Using the foil overhang, lift the bars out of the pan and peel the foil away from the edges. Dust with confectioners' sugar, and cut into 16 bars.